The present invention pertains to the vacuum tube arts, and in particular to a heat barrier for an x-ray tube. It finds particular application in conjunction with rotating anode x-ray tubes for CT scanners and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in the generation of radiation and in vacuum tubes for other applications.
Conventional diagnostic uses of x-radiation include shadowgraphic projection images of the patient on x-ray film or electronic pick-up, fluoroscopy, in which a visible real time shadowgraphic image is produced by low intensity x-rays impinging on a fluorescent screen after passing through the patient, and computed tomography (CT) in which projection images from many directions are electrically reconstructed into a volume reconstruction. A high powered x-ray tube is rotated about a patient""s body at a high rate of speed to generate the projection images.
A high power x-ray tube typically includes a thermionic cathode and an anode, which are encased in an evacuated envelope. A heating current, commonly of the order of 2-5 amps, is applied through a filament or thin layer to create a surrounding electron cloud. A high potential, of the order of 100-200 kilovolts, is applied between the cathode and the anode to accelerate the electrons from the cloud towards the anode. The electrons are focused into an electron beam which impinges on a small area of the anode, or target area, with sufficient energy to generate x-rays. X-radiation is emitted from the anode and focused into a beam, typically through a beryllium window.
The acceleration of electrons causes a tube or anode current of the order of 5-200 milliamps. Only a small fraction of the energy of the electron beam is converted into x-rays, the majority of the energy being converted to heat which heats the anode white hot.
In high energy tubes, the anode rotates relative to the cathode at high speeds during x-ray generation to spread the heat energy over a large area and inhibit the target area from overheating. Due to the rotation of the anode, the electron beam does not dwell on the small impingement spot of the anode long enough to cause thermal deformation. The diameter of the anode is sufficiently large that in one rotation of the anode, each spot on the anode that was heated by the electron beam has substantially cooled before returning to be reheated by the electron beam.
The anode is typically rotated by an induction motor. The induction motor includes driving coils, which are placed outside the evacuated envelope, and a rotor supported by a bearing assembly, within the envelope, which is connected to the anode. When the motor is energized, the driving coils induce electric currents and magnetic fields in the rotor which cause the rotor to rotate.
The temperature of the anode can be as high as 1,400xc2x0 C. Part of the heat is transformed through the vacuum by radiation. Part of the heat is transferred by conduction to the rotor, and to the bearings assembly. Heat travels through the bearing shaft to the bearing races and is transferred to the lubricated bearing balls in the races. The lubricants, typically lead or silver, on the bearing balls become hot and tend to evaporate.
One way to reduce bearing temperatures is to provide a thermal block to isolate the bearing lubricant from the heat of the target. A variety of thermal blocks have been developed for reducing the flow of heat from the anode to the bearing shaft. In one low power design, the rotor stem is brazed to a steel rotor body liner that is then screwed to the bearing shaft. This provides a slightly more thermally resistive path.
Another thermal block that has been used in the industry is known as a top-hat design. A top hat-shaped piece of low thermal conductivity material, such as Hastelloy(trademark) or Inconel(trademark), is screwed onto the hub of the x-ray bearing shaft. The rotor body is then attached to the brim of the top hat with screws, welds, or other fastening means. The thermal conduction path from the rotor body to the bearing is then extended by the length of the top hat. Analysis shows that a 20-50xc2x0 C. temperature decrease may be achieved at the front bearing race when the top hat design is employed. Another thermal block uses a thin molybdenum cone with a highly reflective surface which is pinned to the stem connecting the target with the bearing assembly. The cone follows the contours the target, blocking the view of the target from the bearing assembly. The cone reflects heat radiating from the target, reducing the radiative mode of heat transfer to the bearing assembly.
Another method of reducing heat flow is to use a spiral groove bearing shaft. The spiral groove bearing is a relatively complex, large bearing that employs a gallium alloy to transfer heat. The bearing shaft is limited to a rotational speed of about 60 Hz. This limits operating power of the x-ray tube.
A trend toward shorter x-ray exposure times in radiography has placed an emphasis on having a greater intensity of radiation and hence higher electron currents. Increasing the intensity can cause overheating of the x-ray tube anode. As such higher power x-ray tubes are developed, the diameter and the mass of the rotating anode continues to grow. Further, when x-ray tubes are combined with conventional CT scanners, a gantry holding the x-ray tube is rotated around a patient""s body in order to obtain complete images of the patient. Today, typical CT scanners revolve the x-ray tube around the patient""s body at a rate of between 60-120 rotations-per-minute (RPM). This increased rotation speed has resulted in increased stresses on the rotor stem and bearing shaft. For the x-ray tube to operate properly, the anode needs to be supported and stabilized from the effects of its own rotation and, in some instances, from centrifugal forces created by rotation of the x-ray tube about a patient""s body.
One way to reduce these stresses to a non-critical level is to reduce the length of the rotor stem while increasing the cross sectional area. This, however, shortens and widens the heat conduction path from the target to the bearing shaft, resulting in higher thermal transfer. Recently, x-ray tubes have been developed in which the anode surrounds the bearing shaft, as shown, for example, in U.S. Pat. No. 5,978,447. However, many of the conventional types of thermal radiation blocks, such as the cone design, are unsuited to use in such a configuration, since there is no stem to which a cone may be attached.
The present invention provides a new and improved x-ray tube and method which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, an x-ray tube is provided. The x-ray tube includes an envelope which encloses an evacuated chamber. A cathode disposed within the chamber provides a source of electrons. An anode disposed within the chamber is positioned to be struck by the electrons and generate x-rays. A bearing assembly is surrounded by the anode, the bearing assembly including a stationary portion and a rotatable portion. The rotatable portion is connected with the anode and rotates with the anode relative to the stationary portion during operation of the x-ray tube. A heat shield between the bearing assembly and the anode reduces the radiative transfer of heat from the anode to the bearing assembly.
In accordance with another aspect of the present invention, an x-ray tube is provided. The x-ray tube includes an envelope which defines an evacuated chamber. A cathode is disposed within the chamber for providing a source of electrons. An anode is disposed within the chamber and positioned to be struck by the electrons and generate x-rays. A bearing assembly is concentrically aligned with the anode. The bearing assembly includes a rotating portion connected with the anode by a shaft and a stationary portion thermally connected with a heat sink outside the envelope. A first generally concentric heat shield is between the anode and the bearing assembly. A second generally concentric heat shield is between the first heat shield and the bearing assembly.
In accordance with another aspect of the present invention, a method of operating an x-ray tube is provided. The method includes supporting a rotating anode on a bearing assembly. The bearing assembly is received through a central opening in the anode such that the bearing assembly extends forward and rearward of a center of gravity of the anode. The method further includes interposing a heat shield between the anode and the bearing assembly, operating the x-ray tube such that the anode generates x-rays and radiates heat towards the bearing assembly, and intercepting a portion of the heat radiated from the anode with the heat shield.
In accordance with another aspect of the present invention, an x-ray tube is provided. The x-ray tube includes an evacuated housing and a cold plate mounted to the housing. A cylindrical bearing assembly is mounted to the cold plate. An anode is mounted on the bearing assembly for rotation relative to the housing. A first generally cylindrical heat shield is mounted to the cold plate. The first heat shield extends between and spaced from the anode and the bearing assembly to intercept radiant thermal energy traveling from the anode toward the bearing assembly. A cathode is disposed in the housing opposite to the anode.
One advantage of at least one embodiment of the present invention is that radiative heat transfer from an anode target to a bearing assembly of an x-ray tube is reduced.
Another advantage of at least one embodiment of the present invention is that it centers the center of gravity of the target on the bearing assembly of the x-ray tube.
Another advantage of at least one embodiment of the present invention is that bearing life is increased.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.